The long-term goal of this project is to elucidate the cellular mechanisms of renal calcium transport and its regulation by parathyroid hormone (PTH). PTH-dependent renal calcium transport proceeds in distal nephron segments comprising cortical thick ascending limbs and distal convoluted tubules. The effects of PTH are mediated by the type I PTH receptor (PTH1R). The magnitude and duration of PTH action is ultimately linked to the balance between signal generation and signal termination by its cognate receptor, the Type I PTH/PTHrP receptor (PTH1R). Although the mechanism of PTH1R-mediated stimulation of renal calcium absorption is reasonably well understood, the events involved in and the mechanism by which PTH action is terminated in target kidney cells is not known. The PTH1R exhibits distal nephron cell-specific signaling that involves phospholipase D (PLD) and mitogen-activated protein kinase (MAPK). However, the mechanism by which the PTH1R signals through these pathways is unknown. Finally, although great strides have been made in generating transgenic mice harboring PTH1R or other relevant receptor or calcium channel mutations, it is not known how these proteins function in the intact animal to govern renal calcium homeostasis. Three Specific Aims have been developed to address these deficiencies. Aim 1 will characterize PTH1R internalization, desensitization, and recycling in distal tubule cells. This will be achieved by defining the ligand and receptor structural determinants required for these actions. Experiments will be performed to study internalization and desensitization of wild-type and phosphorylation-deficient PTH1R mutants using incrementally truncated PTH peptides that we show exhibit ligand- and cell-specific induction of receptor internalization. In Aim 2, PTH1R cell-specific signaling in distal tubule cells will be analyzed by determining the mechanism of PTH activation of phospholipase D (PLD); defining the role of PTH-stimulated PLD activity in mitogen-activated protein kinase (MAPK) activation; and characterizing the requirement for PTH1R internalization in MAPK activation. Aim 3 will extend and integrate the questions from the cell and molecular level to an analysis of PTH1R-mediated calcium transport in mice harboring PTH1R signaling mutations, by evaluating the effect of calcium-sensing receptors (CaSR) on PTH-dependent calcium absorption, and defining the role of calcium channel beta-subunits in PTH-stimulated calcium transport. Single microperfused tubules and renal clearance techniques will be applied. The proposed studies will provide novel and important information on the mechanism and role by which the PTH1R regulates renal calcium absorption. The results will provide greater understanding of the initiation and termination of PTH1R action on renal calcium absorption. The outcomes may suggest additional pathophysiological mechanisms causing PTH resistance and lead to new treatment opportunities.